FLEX February 2024

Imaging of Pediatric Head and Neck Masses

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is usually recommended for the workup of a neck mass, assuming no contraindica tions such as allergy or renal impairment. Intravenous contrast is useful for evaluation for abscess, delineating lesion margins, assessment of lesion vascularity, distinguish ing between vessels and lymph nodes, and detection of abnormally enhancing lymph nodes. In particular, cases in which sialoliths or other calcifications are suspected, precontrast imaging through the region of interest is recommended, as contrast may obscure calcifications. In the workup of some thyroid lesions, iodinated contrast should be avoided because of the avid uptake of iodine by the thyroid gland. The advantages of CT in the evaluation of neck lesions are high-resolution delinea tion of anatomy, osseous structures, and airspaces, as well as detection of calcifica tion and fat. It is widely available and provides rapid image acquisition, particularly with newer multichannel scanners, reducing the need for anesthesia compared with MRI. However, sedation may still be required in young patients to minimize motion artifact. Additionally, in contrast to ultrasound, CT imaging is far less operator dependent. The primary drawback of CT is that it requires exposure to ionizing radiation, which is particularly relevant in children, who are more radiosensitive than adults. This is an especially important consideration in patients requiring multiple examinations for long term follow-up. Additionally, there are risks associated with iodinated CT contrast including allergic reaction and nephrotoxicity, which are uncommon but potentially serious. MRI A typical pediatric neck MRI protocol includes multiplanar T1, fat-suppressed T2 or short tau inversion recovery (STIR) sequences, diffusion-weighted images (DWI), and postcontrast, fat-suppressed T1 weighted sequences. 11 Compared with CT, MRI protocols are more complex, and may require tailoring to specific pathologies. T1 weighted images are helpful for delineation of anatomy. Fat appears bright on T1 and T2 weighted images, and fat suppression is helpful for elucidating underlying le sions on T2 weighted and postcontrast sequences. Various fat-suppression sequences are available, which vary by technique and manufacturer, including STIR, iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL), and Dixon. 12,13 Time of flight or time-resolved magnetic resonance angiog raphy (MRA) is recommended for suspected vascular lesions. 14 Diffusion-weighted im aging can increase the conspicuity of lymph node metastases and may play a role in differentiating tumor recurrence from post-treatment changes. 15 High field strength MRI systems such as 3T can provide higher signal-to-noise ratio and/or shorter scan times, which may be particularly advantageous in evaluating small lesions in children. The superior soft tissue contrast capability and lack of ionizing radiation exposure make MRI an ideal modality for evaluating many masses in pediatric patients, partic ularly when intracranial extension is a consideration. 11 However, these advantages must be weighed against the disadvantages of relatively long scan times often neces sitating sedation, as well as the increased cost and resource utilization compared with CT and ultrasound.

CROSS-SECTIONAL IMAGING FEATURES OF PEDIATRIC NECK MASSES Congenital Lesions Branchial cleft cysts

Congenital cystic lesions such as branchial cleft cysts are often well depicted on CT examinations. In some cases, however, branchial cleft cysts and associated sinus

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